Prostate and breast cancers are the second leading causes of death in males and females respectively. Prostate tumors and most breast tumors depend, at least initially, upon circulating hormones for their growth and development. Estrogen antagonists and androgen antagonists comprise the most widely-used endocrine therapies for breast cancer and prostate cancer, respectively. While initial responses to hormone deprivation are the norm, most tumors eventually recur in what is termed a hormone-independent (antagonist-refractory) state. The molecular mechanisms of this resistance are not defined, and this lack of information impedes the rational design of improved anti-breast or -prostate cancer drugs. In the initial application, we proposed the novel concept that hormone antagonists, rather than simply blocking agonist-generated transcriptional and proliferative signals, actually initiate discrete antagonist-specific signaling pathway(s), which both: i.) block AR- or ER-responsive transcriptional responses; and ii.) also act upon the E2F node to block cell-cycle progression. In the course of the first two years of support under this grant, we have discovered a number of novel molecular mechanisms involved in estrogen antagonist- and androgen antagonist-induced transcriptional suppression and growth suppression of breast cancer cells and prostate cancer cells, including class III histone deacetylases, co-repressors, chromatin-modifying complexes, DNA-binding transcription factors, and protein kinases. These components are recruited to hormone-receptor complexes at responsive promoters by antagonists and comprise new signaling pathways activated by hormone antagonists. Loss of any of these signaling components renders the tumor cell refractory to hormone antagonists. In this renewal, in the first two Specific Aims, we will explore the mechanisms through which these factors act upon the antagonist-bound hormone-receptor complexes on responsive promoters to modulate transcription. The composition of the repressor complexes on native, endogenous hormone-responsive promoters will be studied through endogenous ChIP assays, and the interaction of these components with each other on responsive promoters will be analyzed through knock-down, and mutational, analyses. In the Third Specific Aim, the mechanisms through which these signaling pathways converge upon the E2F node to block cell cycle progression will be analyzed. Finally, the mechanistic consequences of natural loss or mutation of these antagonist-signaling components upon cellular responses to antagonists will be elucidated, and correlated with the functions of the components. Collectively, this work will define a new paradigm of nuclear hormone-receptor signaling, and will provide important information relevant to our current approaches to treating hormone-responsive and hormone- refractory tumors, which may be used both prognostically, and for the design of improved therapeutics.